Acids are used broadly in the chemical, electronics, mining and metal finishing industries wherein the acids react with metals to form salts and complexes. Prior electrodialytic processes, see U.S. Pat. No. 4,636,288, provide a satisfactory method for reforming acidic solutions containing multivalent metal salts. These processes comprise electrotransporting multivalent metal cations from the acid through a cation permeable membrane and insolubilizing the metal cation in a catholyte with hydroxyl ions formed at the cell cathode. Unfortunately, metal cations form complexes that have no electrical charge or have a negative charge and the metal cations can not be electrotransported from an acid through a cation permeable membrane. The electrotransport of metal cations through cation permeable membranes also present problems in the prior processes. Multivalent cations form insoluble salts in membranes or on the surface of membranes that reduce or prevent the electrotransport of metal cations. At times, cation permeable membranes are fouled with precipitates when processing acidic solutions that contain only ppm of a multivalent cation that is not successfully transported through a cation permeable membrane into a catholyte. There are few solutions that have no metal cations and most solutions contain cations of two or more metals which increases the problems associated with electrotransport of multivalent metal cations. It would be desirable if acidic solutions containing metal salts could be reformed into acids of the salt anions and insoluble hydroxides of the metal cations without the transport of metal cations through cation permeable membranes. It would be very desirable to be able to reform solutions of acids that contain complexes of metal cations in an electrolytic process without the electrotransport of the multivalent metal cations. It is an object of this invention to provide an electrolytic process suitable for reforming acidic solutions containing complexes and/or salts of metal cations that does not necessitate electrotransport of multivalent metal cations.
Electrodialysis is a well-known art (See U.S. Pat. Nos. 4,636,288; 4,325,792; 4,439,293, the disclosures of which are hereby incorporated by reference.) Electrodialysis is the transport of ions through ion permeable membranes as the result of an electrical driving force. The process is commonly carried out in an electrodialytic cell having an anolyte compartment containing an anolyte and an anode separated by an ion permeable membrane from a catholyte compartment containing a catholyte and a cathode. The ion permeable membrane can be permeable to cations or anions. The anion permeable membrane usually has fixed positive charges and, as the names implies, is permeable to anions and relatively impermeable to cations. The cation permeable membrane usually has fixed negative charges and is permeable to cations.
There are no cation permeable membranes 100% impermeable to anions and no anion permeable membranes 100% impermeable to cations. In all membrane electrodialytic processes there is always some potential for fouling ion permeable membranes by counter ions forming precipitates in and on the surface of the membranes.
There are many complexes of metal cations (See Inorganic Chemistry, Fritz Ephraim, Fifth Edition by R. C. L. Throne and E. R. Roberts ) The complexes can be defined as substances formed by the combination of components which are already saturated according to the classical concepts of valency. The coordination number is commonly six and the complex is not an ion, but is an electrically neutral compound There are no known groups which definitely confer a positive charge on a complex of a metal cation There are, however, substances which can cause an increase in the negative valency and some complexes can have a negative charge. When metals are etched, electropolished, bright-dipped or pickled with acids, such as phosphoric and hydrofluoric, a mixture of complexes and salts of metal cations are formed in the acid or mixtures of acids. The increasing need to protect the environment and to conserve resources make the reuse of the acids and metals desirable. These acidic solutions usually contain two or more acids, two or more metal cations and a mixture of complexes and salts of metal cations and anions of the acids. It is possible to partially reform the acids using electrodialysis (See U.S. Pat. No. 4,636,288.) by removing the metal cations of the salts. However, the anions associated with the metal complex are not reformed and in reuse the concentration of the metal complex increases in the mixture of acids and, at some point, the solution of acids must be replaced or the metal complexes removed. The concentrations of acids and the level of salts and complexes permissible in the acids vary widely in the many finishing processes for metals. These complexities essentially preclude partial reformation of the acids in commercial processes. It is an object of the present invention to provide a process suitable for reforming acids and mixtures of acids in aqueous solutions containing complexes of metal cations or salts of metal cations and mixtures of complexes and salts of cations of one or more metals.